There has been known a technique for reducing ink bleeding caused during printing of an image onto a sheet by ink ejection. FIG. 1 is a schematic diagram depicting a reference example of such a technique. In FIG. 1, a boundary between an area AR1 for black and an area AR2 for color other than black is illustrated. A rectangle PX represents a pixel. Ink color assigned for print processing is represented in each pixel using one or more symbols of K (black), C (cyan), M (magenta), and Y (yellow). For example, “K” represents black ink and “CY” represents cyan and yellow inks. Pixels PXa and PXb are included in the black area AR1. Each of the pixels PXa and PXb is assigned with black ink and represents black. A pixel representing color other than black is present in the neighborhood of the pixel PXa (i.e., a pixel representing color other than black is located adjacent to the pixel PXa). In print processing corresponding to the pixel PXa, C, M, and Y inks are used to represent black in the pixel PXa. Only pixels representing black are present in the neighborhood of the pixel PXb (i.e., no pixel representing color other than black is located adjacent to the pixel PXb). In print processing corresponding to the pixel PXb, K ink is used to represent black in the pixel PXb. As described above, of a plurality of pixels included in the black area AR1, with respect to a pixel (e.g., the pixel PXa) having a neighboring pixel representing color other than black, ink to be used for the pixel is changed from K to a combination of C, M, and Y for representing black. Therefore, one or more of C, M, and Y inks to be ejected corresponding to a pixel representing a color other than black may be avoided contacting K ink to be ejected corresponding to a pixel representing black. Accordingly, intrusion of K ink in a pixel area representing black into another pixel area representing color other than black (e.g., an area onto which one or more of C, M, and Y inks are ejected), that is, an occurrence of ink bleeding, may be reduced.
Nevertheless, in the known technique illustrated in FIG. 1, while the pixels PXa and PXb both represent black, the color of the pixel PXa may be represented by C, M, and Y and the color of the pixel PXb that is diagonally adjacent to the pixel PXa may be represented by K. Therefore, the color of the pixel PXa represented by C, M, and Y may be recognized lighter than the color of the pixel PXb represented by K. If the color of the pixel PXa is recognized lighter than the color of the pixel PXb, a difference in depth of color between the adjacent pixels PXa and PXb may become obtrusive and color may look unnatural. Such a problem may occur not only at a boundary between a black area and a color area but also a boundary between an achromatic color area represented by an achromatic color ink and a chromatic color area represented by three or more chromatic color ink having respective different colors.